Cryocooling systems are employed to cool and transfer fluids, for example, in order to provide the very low temperatures needed for materials research. In such systems, a cryogenic device typically includes portions for lowering the temperature of a fluid to be supplied to the research point, and interface portions for transferring that cooled fluid from the cooling portion of the cryogenic system to the research point. These interface portions may additionally cool the supplied fluid through various methods.
The initial cooling portions of such systems are either open cycle or closed cycle systems. In an open cycle system, liquid cryogen (typically helium or nitrogen) is extracted from a liquid dewar using a liquid transfer line and injected in the cryogenic system to achieve the desired temperature at the sample under test. Cryogen is then exhausted into the atmosphere. This is expensive and logistically difficult, and does not allow long-term operation of the system since the liquid dewar needs to be replaced frequently. In this case however the sample can be remotely located from the liquid dewar and the discharge end of the transfer line inserted into the cryogenic system to provide cooling at the desired location of the sample.
In a closed cycle system, a cryocooler is employed to provide the desired low temperature at the cold stage of the cryocooler. An extension rod or similar setup is attached to the cold station to transfer cold to the cryogenic test sample, which is remote from the cryocooler. This approach has several drawbacks. First, it transmits vibrations from the cryocooler to the test sample. Second, heat load on the extension creates loss of cooling and increases the temperature of the extension rod at the end connected to the point of cooling. This approach also does not provide flexibility in locating the cryocooler, which is typically bulky and creates difficulty in positioning of the system. Such setups also require a large opening in the cryogenic system to insert the cryocooler cold end. These problems become severe when very low temperature liquid helium, such as 4.2 K or below, is desired at the sample.
It is known in the art to provide heat exchange between two or more fluids within the interface portion of a system. Systems of the prior art have typically achieved at least a portion of the cooling process through counter-flow (or parallel) heat exchange between the supply and return fluids of the system. Such systems frequently arrange the transfer lines for these supply and return fluids concentrically, either with the supply line inside or outside the return line, depending upon the relative need for each line to be insulated from ambient temperatures. In such a concentric line configuration, heat transfer, and therefore cooling of the supply fluid, involves convective heat transfer from the fluid that is to be cooled to the first wall of the tube or hose containing it, conductive heat transfer through the first tube wall of that tube to the second tube wall, and convective heat transfer from the second wall of the tube to the second fluid. This results in the desired effect of transferring heat from the first fluid to the second, thereby reducing the first fluid's temperature.
Prior art systems such as those described above rely on precision in the heat balance between the supply and return fluids, as well as selection of transfer line material. Further, the limitations inherent in such method of heat exchange limit the length of the interface over which the cooled fluid may be transferred while maintaining low and precise temperatures.
Therefore, a need remains for an improved thermal interface that permits less interaction between supply and return flows and the associated heat exchange that occurs due to the thermal contact between them, thereby increasing the length over which a cooled fluid may be transferred.